Gas-purification process



Jam. Z, E923.

'E. V. ESPENHAHN. GAS PUR|F|cAT|oN PRocEss.

2 SHEETS-SHEET 1 Fl LED FEB.

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E. V. ESPENHAHN.

GAS PuRrFlcA'rloN PRocEss.

EICRUBBER 'Tow ER GooLE FILED FEB. 9

o Erw G Fun? 2 SHEETS-SHEET 2 51ml. Gcunm Hats OF Damn Feen-new CO? f Nb EN To Emana hun CD3) FunP Patented an.. ,2, i923.

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EDWARD VICTOR EsPENH-AHN, 0F EAST ORANGE. NEW JERSEY, AssIGNoRToTHE KoPPERs ooMPANY, oE-PITTsBURGH, PENNSYLVANIA, A CORPORATION oF PENN- sYLvANIA.

GAS-PURIFICATION PROCESS.

To all whom. it may concern.'

Be it known that i, EDwAR vmfOR Es gas, retort coal gas, water gas, or gas derived 1 from the distillatio-n of petroleum; the invention has for an object to effect an efficient purification ofV such gases, by theabsorption and removal therefrom of suoli constituents as carbon dioxide, sulphur, `for example as hydrogen sulphide, and cyanogen bearing compounds, Whether all or. only one or more of these constituents are present in the gas to be purified. The invention also4 provides a complete regeneration of the gas purifying agent, after such agent has absorbed/the impurities from the gas, the regeneration causing the foul purifying agent to expel ythe absorbed impurities so that they Amay be recovered and utilized industrially, whife the regenerated purifying agent may be returned to' the gas purification operation and used-for further -absorption of impurifies from the gas. This process has the advantage of effecting the regeneration of the purifying agent Without the formation of inert vcompounds which are useless for subsequent gas treatment and 'Would therefore result in Waste of the purifying a ent; consequently', the process comprehen s a. practically complete conservation of the purifying agent, inasmuch as substantially the same volume of the purifying agent may be used 'over and over again with satisfactory results. y

The velimination' of sulphur impurities from fuel gases is a great advantage, The presence of sulphur compounds in fuel gases is obnoxious, especially when the gas is consumed and, ta'void this, it has heretofore been a common practice to'pass the fuel gas through-boxes containing quantities of 'iron oxide' before allowing the gas' to f'ioviv into thesupply mains. Whileiron oxide functions to absorb these deleterious compounds be cleaned frequently and replenished and consequently are expensive to maintain. By

the use of -the present invention, the quantity of iron oxide necessary to insure Vpurification of the gas is reduced to a minimum, inasmuch as the improved purification process' removes from the as substantially all of the sulphur compoun s, as Well as other industri'ally valuable substances hereinabove mentioned, before the gas is allowed to pass into any iron oxide boxes' on itsvvay to the supply mains, i

I-n addition to the general objects recited above, the invention has for further objects such other improvements on advantages in operation and results as are found t0 obtain in the apparatus and process hereinafter describedor claimed.

In the accompanying drawings, forming a part of this specification, and showing for purposes of exemplification' preferred forms and manners in which the invention may beembodied and, practiced,` but Without limiting. the claimed inventionY specifically to such illustrative instance or instances:v

Figure l illustrates a diagrammatic i'e-presentation of apparatus 'for carrying out the improved gas purificationl lprocess of the present invention; and

Fig. 2 illustrates a modified form of apparatus hereinafter more fully described.

In its present embodiment, the invention is applied to the purification of coke oven gas, and for convenience, the present de-A scriptiony'vill be confined to this use of the invention. The features of the invention are, however, susceptible of other -valuable applications, for example to other gases; consequently the scope of the invention is not confined to the specific use and embodiment herein stances as carbon dioxide, hydrogen sulphide described' as an illustrative ex` ample.

and cyanogen bearing compounds from the I gas,- in accordance with the'inventon, there is employed a Water solution containing both an alkali metal carbonate anda bicarbonate. The carbonates and bicarbonates of the alkali earth metals may also be employed in solution or suspension, with or Without admixture of alkali metal. carbonatos and bicarbonates. ,I have discovered, for example, that a solution of sodium carbonate- 'consists of a water solut-ion of the carbonate and. bicarbonate of sodium, which, when the absorbed impurities have been driven off is in substantial equilibrium with the solution entering the gas washer. The ratio of sodium carbonateto sodium bicarbonate in the solution used for\washing the gas is determined by the length of time required to expel the impurities, such as 'hydrogen sulphide, in the regeneration operation. For example,a solution of sodium carbonatebicarbonate mixture that has been used for the treatment of gas containing approximately 1 percentum ofl hydrogen sulph-ide and 2 percentum of carbon dioxide re uires heating for about l5 minutes at 80o in order to expel the hydrogen sulphide to a Sufiicient'extent to restore the solution to its y,originalabsorbing capacity. In such a case, therefore, the gas purification process will be started with a solution of sodium bicar- -bonate that has first been heated'for about l5 minutes at 80"4 C. and then cooled to the 4washing temperature before being allowed to enter the gas washer. Assuming that the solution, before heating, contains 5 percent-um sodium bicarbonate, the heating is effected to convert approximately 8 percentum of the sodium'bicarbonate to sodium carbonate. The percentage of bicarbonateconverted into carbonate becomes less'in more concentratedsolutions, although the total amount converted may be greater. For example, if

- the-original sodium bicarbonate solution be enough carbon dioxide has been absorbed togive the solution the desired composition. However the solution of sodium carbonatebicarbonate mixture be obtained, it is of primfary importance to adjust the composition 0 t e solution so that 1t :corresponds to the solution. Y

composition obtained by heating for a sufficient length of time to expel the hydrogen sulphide. If then this time of heating duringl the regeneration operation is always maintained the same and the gas is fairly uniform withv respect to its content of carbon dioxide,- hydrogen/sulphide and hydrocyanic acid, a condition of equilibrium will be maintained in the solution entering the gas washer. It is desirable to maintain the concentration of the solution 'not less than 5 to 6 percentumsodium carbonate (calculating the'total alkali metal .carbonate as such), the upper limit corresponding to the saturation omt of the solution of sodium carbonate-bicarbonate mixture at the highest temperature at which the 4washing operation is conducted.

When a solution of sodium carbonate-bicarbonate, such asmentioned above, is brought into contact with the fuel lgas to be purified, the following -reactions occur:

There is an excess of sodium carbonate in .the solution before it is brought into condium sulphydrate and -sodium cyanide. By`

subjecting the solution containing the ab sorbed impurities to heat, the solution is decomposed according4 to the following equations which are the reverse ofthe equations of absorption The amount of carbon dioxide present Vin the gases driven off during the decomposition of the foul solution varies according to the amount of vcarbon dioxide absorbed.' Normally, however, the quantitay vof carbon dioxide is at least two or three.y times that ofthe absorbed hydrogen sulphide and hydrocyanic acid. The partial pressure of carbon dioxide .over that in the original gasf has, therefore, been considerably increased and consequently thecarbon dioxide assists the liberation of HZS and HCN by reacting with these substances to form sodium bicarbonate, according to the following equations:

Nans +00, Hzov-Nanoo, 112s NaoN co2 moe-immo, HoN

The bicarbonate formed in the absorption of carbon dioxide from gas is thus decomposed into carbonate and carbon dioxide and the liberated carbon dioxide again reacts with the sodium sulphydrate and sodium cyanide to form other sodium bicarbonate. Part of InV applying the above stated discovery tov the absorption of carbon dioxide, sulphur, and cyanogen bearing compo-unds from fuel gases, such as coke oven gas, the cooled gas after having been subjected to the usual tar and ammonia recovery. rocesses is passed through a gas line 1 (see ig. 1) into a tower or scrubber A in which it flows in counter current with the solution of sodium carbon- -ate-bicarbonate mixture supplied to such 2O scrubber through the solution line 2. In the tower or scrubber A considerably more carbon dioxide than hydrogen sulphide and hydrocyanic, acid is absorbed and the as is consequently. freed of a'large proportlon of its carbon dioxide content. From the scrubber A thegas passes throu h another gas line 3 to a second scrubber in which the gas flows in counter current with fresh solution of sodium carbonate-bicarbonate mixture. Here there is but| relatively little -conversion of sodium carbonate into sodium bicarbonate because of the previous absorption of carbon dioxide froml the gas from the scrubber A, and practically all of the sodium carbonate in the solution is available for the absorption of the hydrogen sulphide and the hydrocyanic acid. The washing solution which enters the scrubber B is supplied through a solution line 4 and the purified gas discharges through a discharge line 5. The solution supplied to both scrubbers A and B is derived from a common solution tank C and the supply line 6 from said tank C is connected at a point 7 with the solution lines 2 and e of the respective scrubbers A and B. Pumps E and F in said supply lines 2 and l maintain a circulation of the solu. tion through the scrubbersA and B.

vFrom the scrubbers A and B the solution discharges through discharge lines 8 and '9 both of which are connected with a common discharge line 10 which leads to the foul solution tank D. The foul solution discharges from the tank D into a foul solution line- 11 and is pumped through said line 11 bl a pump G into an indirect preheater in which it passes in counter-current with regenerated solution conveyed through theregenerated solution line 12 from the boiler or still K. The preheated foul solution from the preheater H discharges through a foul solution line 13 into the topl of a tower or still -solumn I. This still column is of the wellknown type with 'bubbling sections lfor counter current steam' distillation, such as is used for the distillation of ammonia liquor. Thence the solutiongravitates into the boiler or heater K. In the boiler K the foul solution is passed in contact with the steam coils L and is heated thereby to or near the boiling point of the solution. The boiler K is divided into compartments by means of baffles O to ensure uniform travel of the solu tion. The heating of the solution in the boiler K is effective to cause its decomposition according to the equations stated above and the gases of decomposition containing carbon dioxide together with hydrogen sulphide and hydrocyanic acid discharge through the top of the tower I. The gases may be conveyed to a burner for combustion of the hydrogen sulphide to elemental sulphur or sulphur dioxide. The decomposition'of the foul solution in the tankfK requires approximately 15 minutes and when effected, the regenerated solution of sodium carbonate-bicarbonate mixture is pumped by a pump M through a solution line 12 to the preheater H in which it is cooled during its passage in counter-.current with/the foul solution supplied by the foul Isolution line 11. Thence the regenerated solution discharges through cooling coils N in which it is further cooled and finally passes through the solution line 15 to the solution tank C for subsequent separate delivery to the scrubbers' A-and B in which the regenerated solution is again used in the absorption operation.

.l'nstead of dividing the solution from the tank C into two parts,"l as above described. and causing it to flow in parallel through the scrubbers A and B the solution may be passed in series through said scrubbers. For this purpose, a bypass 16 connects the solution line 2 with the discharge line 9, said by-pass being provided with a valve a. There is also a valve in the line 7 below the by-pass 16 and a Valve c in the discharge line `9 of the scrubber B. By closing the valves c and b, the solution may be first' circulated through the scrubber B in which it comes in contact with the gases containing the least carbon dioxide and thence passes from the scrubber B into the scrubber A where the major portion of the carbon dioxide absorption is effected. Thus the fresh solution, ,which contains more carbonate available for the absorption of hydrogen sulphide and hydrocyanic acid is brought first into contact with the gas in the second lOO employed for this purpose, it is preferable to first remove the sulphur dioxide from such gases.

Referring to Fig. 2 the revivified solution from the tank C is fpumped from the solutionV in the collecting Atank D. The distribution of solution into the two scrubbers and the admission of furnace gases into scrubber W are adjusted in such a Way that sufficient excess sodium bicarbonate is contained in the mixed solution in the collecting tank to effect the reactions in the solution regeneration operation, as Well as to maintain the proper proportion of sodium bicarbonate in the solution entering the scrubber A. Other- Wise the process is carried on in exactly the same manner as has been described in connection with Fig. 1. f

In' each of its forms, the invention excludes the outside air entirely 'from contact with the solution of sodium carbonate-bicarbonate mixture. Consequently no oxygen, except the slight traces that may be present in the gas to be purified, `can come into contact with the solution. By preventing access of the solution to free oxygen or air, the formation of sodium v thiosulphate is prevented. This is an important advantage, for the reason that the formation of sodium thiosulphate involves a Waste of soda, thiosulphate being so muchinert material. The invention thus accomplishes a substantially complete conservation of the gas purifying solution.

The invention as hereinabove set forth may be variously embodied Within the scope of the claims hereinafter made.

l. Iii a process for purifying gases, the combination-of steps that consists in: passing the gas throu h a solution of an alkali 'metal carbonate-bicarbonate mixture to absorb the impurities from tliegas, subjecting thesolution ycontaining the absorbed im .purities to heating to remove such impurities from and to effect a regeneration of such solution, and utilizing the rejuvenated (solution for further gas purification; subi impurities from the gas, subjecting ,the

to heating to remove such impurities from and to effect a regeneration of such solution, and utilizing the rejuven'ated solution for further gas purification; substantially as specified.

3. In a process of purifying gas, the combination of steps that consists in: passing the gas through a solution of an alkali metalcarbonate-bicarbonate mixture to absorb the impurities from the gas, then heating the solution containing the absorbed iinpurities to expel such impurities and continuing the heating until the rejuvenated solution is in substantial equilibrium with the solution going to the gas purification operation, and utilizing the sorejuvenated solution for further gas purification; substantially as specified.

4. In a process of purifying fuel gas, the combination of steps that consists in: passing the gas through a solution of sodium carbonate #bicarbonate mixture to absorb the impurities from the gas, then heating the solution containing the absorbed impurities to expel such impurities and continuing the heating until the rejuvenated solution is in substantial equilibrium with the solution going to the gas purification operation, and utilizing the so i'ejuvenated solution for furtherl gas purification.; substantially as specified.

5.#In a process of purifying fuel gas, the combination of steps that consists in: passing the gas through a solution of an alkali metal carbonate-bicarbonate mixture to al)- sorb the impurities from the gas, then expelling such impurities from the foul solution to effect a rejuvenation thereof, the rejuvenated solution being maintained in substantial equilibrium with the solution going to the ga-s purification operation, and passing the rejuvenated solution to the gas purification operation; substantially as specified.

6. The improved gas purification process consisting in maintaining a continuous circulation of a solution of an alkali metal carbonate-bicarbonate mixture in contact with the gas to purify the latter and thence to a heating operation, combined with continuous cooling and recirculation of the solution back to the gas purification step; sub- ,.stantially as specified. 7. The improved gas purification process consisting in maintaining a, continuous cir-- culationv of a solution of an alkali metal carbonate-bicarbonate mixture in contact with the gas to purify the latter and thence to a heating operation; combined with continuous cooling and recirculation of the solution .back to the gas purification step, the entire volume` of solution in circulation being substantially free from contact with free oxygen; substantially as specified.

' 8. Ina process of purifying fuel gas, the

combination of steps that consists in: first passing the gas through a solution of an alkali metal carbonate-bicarbonate mixture to absorb the carbon dioxide from the' gas, then subjecting the gas to afurther absorption process with alkali carbonate-bicarbon- .ate solution to remove from the gas the hypurification; substantially as specified.

9. In a process of purifying fuel gas, the

'combination of steps that consists 1n: first passing the gas through a solution of an alkali metal `carbonate-bicarbonate mixture, to

absorb the carbon dioxide from the gas, then subjecting the gas to a further absorption process with alkali .carbonate-bicarbonate solution to remove from the gas the hydrogen sulphide and cyanogen bearing compounds, collecting the solution from both absorption steps and subjecting it to heat to remove the impurities from and effect a regeneration of such solution, the rejuvenated solution being in substantial equilibrium `with the solution going to the gas purification operation, and utilizing the rejuvcnated solution for further gas purification; substantially as specified.

l0. 1n a process of purifying fuel gas, the combination of steps which consists 1n: subject-ing the gas to two absorption steps, one principally for the absorption of carbon Clioxide and the other principally for the absorption of hydrogen sulphide and cyanogen bearing compounds, by passing the gas in one sta-ge through a solution of an alkali Inetal carbonate-bicarbonate mixture and in the other stage through fresh alkali metal carbonate and bicarbonate solution, collecting` the 'solution from both such absorption stages and subjecting such solution to heat to removethe impurities from and effect a regeneration of the solution, and utilizing the` rejuvenated solution for further gas purification; substantially as specified.

EDWARD VICTOR ESPENHAHN. 

